w = 3; //mm
h = 20; //mm
E = 40; //mm
n = 20;

tolerance = 1; //mm

theta = 2*atan(w/(2*h));
alpha = n/2*theta;

R = sqrt(h^2+w^2/4);

L = 2*R*sin(alpha)-tolerance;
P = R*cos(alpha);

module comb(){
    union(){
for ( i = [0 : n-1] ){
rotate([0,0,-alpha+theta*i+theta/2]) translate([-w/2,0,0]) cube([w,h,L],false);
}
}
    }

difference(){
translate([-L/2,0,0]) cube([L,E,L],false);
    translate([0,-P,0])
comb();
    translate([0,0,L/2]) rotate([0,90,0]) translate([0,0,-L/2]) translate([0,h+E+P-h,0]) 
    rotate([0,0,180]) comb();
}